build: don't mix up package and executable names

Makefile

@@ -75,7 +75,7 @@ ifeq ($(TARGET),gtk)
   ifeq ($(GTKFLAGS),)
     $(warning $(PKG_CONFIG) can't find gtk+-3.0, or $(PKG_CONFIG) itself can't be found)
     $(warning if you have the needed files installed, specify their locations and names with `make GTKFLAGS='-I/usr/include' GTKLIBS='-L/usr/lib -lgtk'`)
-    $(warning if not, the package names under Debian and derivates are '$(PKG_CONFIG) libgtk-3-dev'; for other distros, consult a search engine)
+    $(warning if not, the package names under Debian and derivates are 'pkg-config libgtk-3-dev'; for other distros, consult a search engine)
     $(warning If you instead want to build the CLI version, set the TARGET environment variable like so:)
     $(warning TARGET=cli make)
     $(error Can't build gtk target without gtk dependencies)